As the life expectancy of older Americans increases, prevention of age-associated physical function decline and disabilities has emerged as a major clinical and public health priority. A critical factor in an older person's ability to function independently is mobility, the ability to move without assistance. Older people who lose mobility are also less likely to remain in the community, have higher rates of morbidity, mortality, and hospitalizations and experience a poorer quality of life. While several studies suggest that physical exercise may prevent physical disability, including mobility disability in healthy and frail older adults, definitive evidence is lacking. A Phase 3 randomized, controlled trial (RCT) is needed to fill this evidence gap. Preliminary data to estimate sample size needs for such a trial, however, are currently insufficient, and further feasibility data should be gathered before such a trial can be effectively designed and implemented. In response to specific guidance from the NIA that relates to key trial design benchmarks (including sample size calculations to demonstrate the feasibility of a full-scale trial and refining/developing recruitment, procedures, materials and organizational infrastructure), we propose to conduct a pilot, single-blind RCT involving comparison of a physical exercise program of moderate intensity with a health education control. A total of 500 sedentary persons aged 70-85 years who are at risk of disability will be followed at 6 sites for greater than or equal too 1 year. We will assess the combined outcome of major mobility disability defined as incapacity to walk 400 m, or death, which will be the primary outcome of the full-scale study. To our knowledge, this outcome has not been used in previous RCTs, and therefore, a pilot study is needed to assess its incidence rate. Secondary outcomes will include ADL disability, major fall injuries and cardiovascular events. We will explore the effects of the intervention on physical performance measures, cognitive function, health-related quality of life, and use of health care services. In addition, we will perform cost-effectiveness analyses of the intervention. This pilot study will yield the necessary preliminary data to design a definitive Phase 3 RCT. By providing a conclusive answer regarding whether physical exercise is effective for preventing major mobility disability or death, the results of the full-scale trial wilt have relevant clinical and public health implications, and will fill an important gap in knowledge for practicing evidence-based geriatric medicine.